Frequency-translating system



Get. 6, 1925.

L. ESPENSCHIED FREQUENCY TRANSLATING SYSTEM Filed Oct. 25. 1922 NN. A .a

A TTORNEY UNITED"is'iui'rasv Patented Oct. 6, I

PATENT A caricav LLOYD ESPENSGHIED, 01E-HOLLIS, NEW YORK, ASSIGNOB-TO AmIOAN illIaIPHON v -ANTI) TELEGBAPH COMPANY, A OQBPOBATION l' NEW You l irnaoUENcx-raanmrme ss'rml.

Application Med October 85, 1822. Ierhl Io. 598,888.

To all wlizom t myzcloncem:

Be it known that I, LLOYD EsrENsomnD,

residing at Hollis, in the. county of Queens 'and State of New York, have invented certain Improvements in Frequency-Translating Systems, of which the following is a specification. 5

This invention relates to wave transmission systems, and more particularly to trans-v mission of the energy of given waves or 1mpulses by means of waves of lower frequency than those whose energy is to be transmitted.

v An important field of electric transmission is that involving the use of unidirectional or alternating currents modified in certain of their characteristics, such as continuity, intensity -or frequency. InI this field, which includes electric signaling and other teledynamic electric operation, efficiency from an energy standpoint is important, but is frequently much less important than faithful transmission of the wave forms involved. In the case of telephony there is involved transmission of a wide rangeof essential frequencies, and it is desirable that these frequencies be transmitted without distortion and without utilizing unduera-nge of the frequency spectrum of the transmission medium employed.

In accordance lwith the method of the present invention these difficulties are overcome by the reduction of the effective frequencies of the currentsto be transmitted, so lthat they may be transmitted over circuits of limited frequency range, or, in the case of transmitting media having larger frequenc ranges, without utilizing an unnecessariiy wideband of the frequency range available. Since the transmission of speech involves waves extending through a. wide band of frequencies, it is necessary, if the intelligibility andv naturalness of the speech are to be preserved', to make the transmissionv such as to retain, so -far as possible, each of the initial frequency elements.

'Where' the highest essential frequency elef ments exceed the upper limit of the frequency transmission range of the circuit, successful transmission thereover is not pos-l sible. The object of the present invention is to effectively reduce the frequencies of the .various com nents of a band of currents sol as to ma e them fall within the transmission range of commercial, transmission circuits.

In the case of certain types of signaling bands, such, for example, as a band of telephone frequencies, it may not be necessa or even desirable to transmit all of the frequencies within the limits of the band. In

other words, the original band may be thought of as not being continuous but con- 'sisting of a number of smaller subbands separated b blank or unessential frequency intervals. ere such is the case, it is proposed, in accordance with the resent invention, to so o 'erate upon the cwest freuency'subban as to subtract from the requency of each of its components the original frequencyof its lowest frequency component, thus reducing the subband to a range of fre uencies from zero to a new upper limiting requency. At the same time there may be subtracted from the frequencyl of each component of the second or next highest subband a frequency sufficient to reduce its lowest frequency component to approximately the new upper limiting frequency of the first subband. The third subband may be similarly reduced in frequency so as to make its lowest frequency approximately that of the highest frequency o the reduced second band. A similar treatment may be applied with respect to each subband with the final result that a continuous band will be obtained extending from zero to a fixed upper limit much lower than the upper limiting frequency of the original band of frequencies. This continuous band may be transmitted, and after transmission may be separated into its vat rious subbands. and each subband may be stepped up in frequency to its original frequency position.

By treatinga band of si aling fre uencies in the manner previous y describe the range of frequencies heretofore necessary for the transmission of telephonie signals,

for example, may be encompassed for trans-y mission purposes within a very much narrower frequency range without any inherent distortion in the signal itself. This not only enables the use of a circuit having rela-- s component frequencies each reduced by the' tively sinall frequency range for the purpose ott transmitting a signal involving a much wider frequency ran e, but also renders it possible to so eiciently utilize the fre uency spectrum available in the case of a ra io or wire carrier system' as to per#v init of superposing a larger number of caru ceiving instrumentality, In other words,v

the method of the'present invention is essentially one in which a high degree of secrecy is obtainable.

ln orderto understand how the results of the present invention are obtained,'it should be remembered that the modulation of a carrier wave by waves of a band of fre uencies is to produce two so-called side ands. See, for example, the article entitled Carlrier current telephony and telegraphy? by' Eolpitts and Blackwell, pages 307 to'310 inclusive, Journal of the American Institute of Electrical Engineers, vol. 40, April 1921,

No. 4. Of these two bands the upper sida bandmay be considered as the original band of waves having the fre uency of each of its components increased y the carrier frequency.v Since instead of a high carrier fre quency, there maybe impressed upon the modulator a-frequency c anging wave of any definite frequency, it is obviously possible to increase the eective frequencies of each component of the band by this definite amount. lf the'fi'equency of the Wave applied to the modulator is lower than that of the lowest component of the original band, the lower or difference frequency side band resulting from the modulatin action may be considered as theoriginal and with its definite frequency. There are accordingly available simple methods of stepping up the frequency of bands of waves or stepping them down at will.v

The operation of both modulators anddemodulators may be looked u onA as that of frequency changes. The modu ator operates to change the frequency bythe fre uency of the so-called carrier wave applic thereto.

' The demodulator in general changes the fre- 4quency of a band o ponents bythe frequency of the unmodu modulated wave comlated carrier wave`component which is usually .present with the side bands. In both instances upper andv lower side bands are produced by the frequency changing operation, and it is necessary to s ele'ct the desired instaan side band by tive circuits.

With these. principles in mind, it will be obvious that the results sought by the present invention may be obtained b selecting the essential subbands of the widerA frequency band to be transmitted in the individual circuits translatin the frequencies of the subbands by moduiting them with carrier fre- (uencies so chosen as to step each subband down to such apositionin the fre uenc spectrum that all of the subbands will o cupy a narrower frequency ran e extendin from zero up to the maximum requency o the upper step ed down subband. `The waves may now e transmitted in this condition, and at the receiving end of the sys-I tem' the component subbands may be againf means of filters or other selecselected into individual circuits by means! of suitable filtersand by a process of modulation each; subband may be again restored y to its ori inal-iosition inthe fre uenc s ec# l L Y trum. 'l erestored subbands producesubstantially the saine-eifect, except en app ied to an lordinary receiving instrument will` '90 for some possible loss of articulation,- as

though the original frequency band had 7 been applied to the 'receiving instrument without undergoing any frequency translai' tion. v

The invention may now be more fullyv understood from the following detailed description thereof when read in connection with the accompanying drawing, 'the figure of which illustrates in a simplifiedfiand schematic forni the circuit elements necessary to carry lout the process of the invention.

Referring to the drawing, L designate-S.

an ordinary telephone transmission line ter# minating in the usual form of subscribers substation set. At the other end the line terminates in the usual type of hybrid coil 10 and balancing network N. A' common" transmitting circuit TL and a common ceiviiig circuit RLinay be associated wit the line L through the hybrid coil 10, and

individual transmitting and receiving chan-` nels are associated with the main transmitting and receiving circuits TL and RL, as 4indicated by the characters TL1, TL2, etc., which designate the individual transmitting channels, and thecharacters RLl, RL2, etc.,

designate the individual receiving channels. Thel individual channels at their opposite ends are again combined in the common transmitting and receivin circuits TL and RL, which are associate by means of a hybrid coil 11 and balancing network MN with the main transmission line ML. The subscript 1 applied to a ypiece of apparatus or al channel v46in the drawing indicates that that piece -of apparatus or channel is used i-n connection with the lowest subband of the lar r band of frequencies to be transmitted. v imiliarly, the subscript '2 has refertype disclosed ence t' a piece of apparatus or a .channel utilized in connection with the next lowest subband, and the subscript n refers to a piece of apparatus or a channel used in connection with the highest subband of thev A however, occupy a much narrower range main bandto be transmitted. l

In order to select the lowest transmitted subband into the channel TL1, a filter F1 is provided, said filter being, for example, of the well known Cam bel type and having cut-0E points coinci ng wit the limiting frequencies of the lower subband. For the purpose of transmitting the selected band of frequencies into a range extending from zero upward, a modulator M1 is piovided This modulator may, for example, of the in Carson, Serial No. 1,343,307, of June 15, 1920. The modulator is provided with a carrier frequency equal to the lowest `fr uency of the selected subband through a lter S in a manner to be described hereafter, and by beating the selected band with said carrier frequency, two side bands are produced, an upper and a lower side band. The lower side band will include frequencies corresponding to the individual frequencies of the original subband selected, but these `frequencies, while extend-` ing over a range of equal width, will extend from zero upto a finite frequency, instead of from one finite frequency to another finite frequency. In order'to select this lower band from the upper side band, a filter G1 is provided. This filter may be of the Campbell type, and .its cutoff' points are s0 chosen that it will transmit a band extending from zero upto the upper limit of thetranslated subband of frequencies. In a similar manner, the transmitting channel TL2 is provided with filters F1; and G2 and a modulator M1'. These pieces of ap aratus' are similar to those already described in connection with the channel TL1, except that the lilter F2 selects the next important subband of frequencies above the subband selected by the iilter F1, and the filter G2 selects a band of the same width as that selected by the filter F 2, but extending upward from the upperlimit of the band selected by the lter G14 instead of extending upward from the lower .limit of the original subband selected into the lchannel TL1. The modulator M2 is provided with a carrier frequency `through the filter T, the frequency of said carrier being so chosen as to produce a lower side bandv corresponding to the transmission range of the filter G2. In a similar manner thetranslter Fn selects the upper component subband of the main band to-be'transmitted. The modulator .Mn steps itA down in frequency, and the iilter Gu selects the stepped down frequency subband. Consequently there will be impressed upon the main line ML through the hybrid coil 11 a frequency band having frequencies corresponding tothe essential frequencies in all of the main component bands of the orivinal band-to be transmitted. This stepped own band will,

than that of the original band.

T he receiving channel corresponding tol each transmitting channel will ineludeliltf ters having the same characteristics as cor-v responding filters in each transmitting chan-` nel, and in addition, the demodulators D1,

D1 and n will be provided in the receiving channel.' The vfilters G1, G2, ete-,"select the component subbands in 'the translated frency band transmitted over the main line and limpress the individual translated band selected upon the corresponding demodulators. Each demodulator, which may be of the ty e disclosed in Carson Patent 1,343,308 of une-15, 1920, has, applied to its input circuit a carrier frequency which is applied to the demodulator. This side band,

lwhich corresponds to the originalsubband of the main band transmitted at the distant station, will be selected by the band filter' F1 or F1, etc., as the case maybe. 'Consef quently, stepped up subbands corresponding to the essential subbands of the original messages will be combined in the receiving circuit RL and will be transmitted to the subscribers line L.

While individual sources may be einployed for supplying the carrier frequency to each modulator and its corresponding .de-

modulator, it is preferred, where possible, to ein loy aharmonic generator, schematically indicated at HG, for the purpose of producing carrier frequencies which are harmonics of some funda-mental base frequency, the desired harmonics being selected and transmitted to the proper modulator and demodulator by means lof iilters such as'S, T and V. These filters are preferably simple tuned circuits of a type well known' in the art. The harmonic generator HG is also a well known piece of apparatus and is fully described in the Colpitts-Blackwell paper previously referred to. v

The apparatus so far described may be located at one terminal-of the main transmission line ML, or other medium 4which isY employed for transmission between two --distant points. At the distant end asimilar equipment will be provided as indicated in the drawing, its corresponding elements be- 'represented by the same reference char- A minal line L, therefore differs from the bandy transmitted over the main line of a carrierl 'i creasey in the frequency without any attemptv acters as those already described, except that the reference characters are primed.'

The operation of the circuit asal whole will, of course, be obvious from description of the operation alread given in connection with the elements of t e circuit. lt will be noted, however, that for ,transmission from the substation set of the line L to the substation set of the line L a normal band of frequencies will be transmitted over the line L and translated into 'a narrower band of lower frequency before being a plied to the main line VML. The band o -fre uencies translated over the main line ML, eing a narrower band and of lower frequency than the original band transmitted over the tertransmissioncircuit, as in the latter case the translation 1s upward with a consequent 1nbeing made to narrow the band corresponding to a given signaling channel. The narro'wed band transmitted over the main line translated into a band of fre uencies corre-v sponding to the original ban in a manner obvious from the description of the receiving apparatus already given, the expanded band being transmitted over the line L to the distant subscribers set. lt will also be observed that the band 'of fre uencies transmitted over the line ML will, ecause of the translation in fre uency and because of the narrowing of the intelligible, thereby rendering it dilicult for unauthorized persons to tap the circuit.

The system as described has reference to the transmission of the signal from a sin le channel over a main line such as ML. e scheme is a plicable, however, to either carrier transmission over wires or carrier transmission by radio. In either of these cases the narrowed band,` which in the case illustrated is applied to the line ML, will be applied to the modulating channel of the wire carrier or radio carrier a paratus, and at the receiving station the and detected by the demodulator of the carrier system, whether wire or radio, will corres ond to the narrowed band received from t e-main line ML in the case described. Consequent- 1y, the detected band in the carrier system will be impressed u on a receiving arrangement like that already described. As the 1nvention proper is entirely independent of the type of transmitting medium employed in lieu of the line ML, it is unnecessary to v illustrate the. same in connection with carrier equipment.

It will be obvious that the general principles herein disclosed may be embodied in many other organizations 'widely different continuous band o .an extent lying and be substantially un' -selected elements a continuous y masacre from those illustrated without departin from the spirit ofthe invention as define in the following claims.

yWhat is claimed is:

l. The method of transmitting a band of waves over a conducting ath which offers an excesslve transmission oss to the higher frequency components thereof which method comprises reducing the frequencies of all of the components to frequencies well within the eiiicient transmission range of said path. 2. The method of transmitting the essential'elements of a broad bandlof waves over a circuit having a ractical transmission range of more limite extent than said band which comprises selectin the essential ele`- ments of said band pro ucing therefrom -a Within the transmission range of said circuit, transmitting said continuous handover said circuit and restoring each of the elements thereof to its origlnal frequency position.

3. The method of transmitting a band of waves lying between two definite frequency limits over a conductor within the transmission range of which one of said limits does not fall which method comprises reducing the frequencies' of each component of said band by predetermined amount to cause said frequency limits to fall within said transmission range, transmitting said reduced frequency `band over said conductor and thereafter increasing the frequency of each of the transmitted com onents by the sameY 5. A transmission system for transmittin a band of waves over a conductmg pat which offers excessive transmission loss to the higher frequency components thereof, comprising means for reducing the frequencies of all ,of the components lof said band to frequencies well within the efiicient transmission range of said path.

6. Al system for transmitting the essential elements of a broad band of waves comprising a circuit having a practical transmission range of more limited extentthan said band, means for selecting the essential elements of said band means for producin from said and of an extent lying within the transmission ran e of said circuit, means for transmitting sald continuous band over said circuit, and means for restoring each of the elements 'transmitted thereover to its original frequency position.

7. In a system for transmitting a band of Waves lying between two definite frequency limits over a conductor Within the transmission range of which one of said limits does -not fall which comprises means for reducing the frequency of each component of said band by la predetermined amount to cause said frequency limits to fall within said transmission range, means for transmitting said reduced frequency band over said conductor to a distant station, and means at said station for increasing the frequency of each of the transmittedl components by the same predetermined amount by which it was .reduced before transmission.

8. The method of electrically transmitting intelligence which consists in lreducing the frequency range represented by the signal currents, transmitting the reduced range of frequencies over a translating medium and restoring the transmitted range of frequencies tothe original range of frequencies.

9. The method of electrically transmitting intelligence, which consists in selecting the essential component subbands of a main band of frequencies corresponding to a signal, translating the frequencies of the selected subba-nds to produce corresponding bands which are contiguous, selecting the component bands. after transmission and. restoring the frequencies of the selectedsubbands to those of the original subbands to produceL a range of frequencies corresponding to the ori inal range.

n testimony whereof, I have signed my name to this specification this 24th day of October, 1922. y

LLOYD EsPENsoHIED. 

